1. Field of the Invention
This invention relates to a projection lens and, more particularly, to a projection lens for projecting an image displayed on a projection tube for use in a projection television system.
2. Description of the Prior Art
In a projection television system, three primary color images of red, green and blue displayed on three projection cathode-ray tubes are projected on a screen by means of three projection lenses for displaying a large-sized color pictures. In such projection television system, it has been required to adopt a high performance projection lens having a large angle of view, large aperture ratio and high capability of image formation in view of providing a thin and compact projection television receiving set.
Consequently, there have been proposed many kinds of projection lenses in the past, such as a projection lens composed of all glass component lenses to maintain high quality through tight machinery tolerances, a projection lens composed of all plastic component lenses to obtain a large aperture ratio as well as to decrease the cost of production and a hybrid projection lens composed of glass component lenses and aspheric plastic component lenses.
However, there have been some problems in the prior-art projection lens, such as of all glass component lenses, that if an imaging power is increased while maintaining a large aperture ratio, a number of component lenses should be increased, thus resulting in the high cost of production and, to make matters worse, a performance at the peripheral portion of the lens will be decreased rapidly.
Further, as to the prior-art projection lens such as of all aspheric plastic component lenses, there have been different problems such that the tight machinery tolerance is not expected like in the machinery for glass component lenses, this in turn makes the manufactured lens unable to perform the intended performance as designed, and thereby causing a change in the refractive index, or even in a shape, of component lenses due to a change of temperatures, thus resulting in a displacement of a focal point for decreasing the imaging power.
It is therefore an object of this invention to eliminate the problems encountered by the prior-art projection lenses and to provide a projection lens not increased in number of component lenses in spite of having a large aperture ratio, not decreased in peripheral performance and that not decreased in imaging power by a change of atmospheric temperatures.